1. Field of the Invention
This invention relates to a method and apparatus for accurately measuring areas of photoelectric cells and photoelectric cell performance parameters, and more specifically, for determining and measuring an effective electric current-producing surface areas of solar cells, and how much current is produced in these areas, as well as creating visual and numeric representations of an efficiency of solar cells.
2. Description of the Prior Art
It is often desirable or necessary to determine and evaluate electric current produced by photoelectric cells, such as photovoltaic devices or solar cells, as well as the energy or power conversion efficiencies of the solar cells in order to specify solar cell performance, evaluate quality, or classify them for scientific, engineering, or economic feasibilities. One performance parameter or variable utilized to specify energy conversion efficiency is the electric response in terms of current produced per unit of area of the solar cell. This particular parameter is often measured as current density which can be defined as the total short circuit current J.sub.sc (zero load resistance) divided by a cross-sectional area perpendicular to the direction of the current. To accurately quantify the current density, it is necessary to determine and measure the exact surface area of the cell.
Prior to this invention it was often very difficult, if not impossible, to determine the exact surface area of some solar cells for a number of reasons. For example, some solar cells are so small that it is difficult to get sufficiently accurate size measurements of the cells with conventional measuring instruments. Further, and perhaps of more significance, the fabrication of p-n-p or p-i-n junctions in many solar cells dictates using several layers of materials which typically results in such a solar cell base layer that is wider than the layer considered to be the light-absorbing surface.
For example, a common method of fabricating amorphous silicon (a-Si) solar cells is to use stainless steel as a substrate back contact for p-i-n amorphous silicon (a-Si) structure and a transparent conductor, such as indium tin oxide(ITO), to define the cell area. While most of the electric current is produced in the defined cell area, there is also some current collection or production beyond this geometrically defined cell area. Another example of a solar cell in which the effective cell area is not always clearly defined is a common cadmium zinc sulfide/copper indium diselenide structure. The solar cell area of such devices is usually considered to be the top, layer. However, electric current collection from outside the top layer can, and often does, occur if carriers generated in the underlayers can reach the edge or junction.
Consequently, it is frequently very difficult to quantify the exact value of the current density, and thus also specify the exact solar power conversion efficiency of a solar cell with conventional measuring techniques. In this regard, solar cell power conversion efficiency N is often defined as the ratio of the maximum power P.sub.m to the product of imput solar power or radiation intensity I multiplied by the cell area A. ##EQU1##
P.sub.m is generally defined as the product of the short circuit current density J.sub.sc, V.sub.oc, and FF. V.sub.oc is the open circuit voltage of the cell, and FF or fill factor is the ratio of the product of the maximum current I.sub.m and maximum voltage V.sub.m to the product of V.sub.oc and J.sub.sc. J.sub.sc is the short circuit current (zero load conditions) of the cell.
All of the performance parameters except for J.sub.sc normally can be measured with standard terrestrial photovoltaic measuring equipment available for that purpose independently of cell area. In this regard the power conversion efficiency value N obtained for a given or constant current density J.sub.sc and the remaining performance parameters will vary in an inverse proportion to the cell area measurement used in the calculation. Thus, if the area value used in the calculation is smaller than the actual effective light-absorbing area of the solar cell, the conversion efficiency N will appear to be greater. Conversely, use of a smaller area in the calculation will result in a smaller conversion efficiency value. Consequently, any accurate conversion efficiency determination requires an accurate measurement of the cell collector area from which electric current is produced.
At present, the definition of cell area most commonly used by skilled persons working with the testing and evaluation of photoelectric (solar) cells is "the entire front surface area of the cell, including the area covered by grids and contacts." This conventional definition is normally inadequate in measurement procedures such as those discussed above where the geometry of the solar cell device allows electric current collection from an area beyond the portion of the device generally considered to be the light-absorbing cell surface. The conventional definition is also inadequate in the sense that it includes the area covered by conductor grids and contacts within the cell. In reality the latter areas do not function as conversion materials in the cell. Further while using the conventional definition to determine and quantify solar cell efficiency, it typically provides a conversion efficiency value that is averaged over the entire measured area but, it does not usually provide a qualitative analysis or detection of spot impurities or less productive zones within the measured cell area. measured.